Transmission help!!!!

I was driving my 2000 ss Camaro it's automatic when I put it full throttle and it gets up in the powerband and goes to shift it makes a loud clunk and acts like it's in neutral tell I let out then it engages and works normal I have the diablo sport programmer on it with the shift pressure turned up a little what would cause this any help please!!!!

Trans has 78k I haven't changed the fluid iv only had it about 5k miles I jumped the shift pressure up 3psi and shift quickness down 3 and jumped the shift points up a little my first thought was valve body if I just drive normal it's fine the fluid is a little low but not bad I drove it for about 5 hours today and did fine all day